docs(cactus-core-api): add README file

Depends on hyperledger-cacti#1556 - feat(core-api): add weaver protobuf codegen

Fixes hyperledger-cacti#833

Signed-off-by: Peter Somogyvari <peter.somogyvari@accenture.com>

packages/cactus-core-api/README.md

@@ -0,0 +1,73 @@
+# @hyperledger/cactus-core-api
+
+This package is meant to be depended on by most of the other packages for interface
+definitions, abstract classes and generated code. All of which is to be kept with
+as few dependencies as possible in order to reduce the potential for circular
+dependencies.
+
+From the above it also comes that the `core-api` package is rarely used by developers who are implementing projects with Hyperledger Cactus and instead it is mostly consumed by the framework's other packages internally (e.g. the Hyperledger Cactus contributors).
+
+# Usage
+
+## Installation
+
+You can install the package via your favorite package manager.
+
+Yarn: 
+
+    yarn add --exact @hyperledger/cactus-core-api
+
+npm: 
+
+    npm install --save-exact=true @hyperledger/cactus-core-api
+
+> We highly recommend using exact versions in general when managing your dependencies
+> in order to achieve (or get closer to) [reproducible builds](https://reproducible-builds.org/) and to enhance your
+> security posture against malicious package versions that might get pushed to the
+> registries without the knowledge or consent or well intentioned maintainers.
+
+## Import
+
+```typescript
+import {
+  GetKeychainEntryRequestV1,
+  isICactusPlugin,
+  IEndpointAuthzOptions,
+  IExpressRequestHandler,
+  IWebServiceEndpoint,
+} from "@hyperledger/cactus-core-api";
+```
+
+## Check If POJO is a Cactus Plugin
+
+```typescript
+if (!isICactusPlugin(plugin)) {
+    throw new Error(`PluginRegistry#add() plugin not an ICactusPlugin`);
+}
+```
+
+# Development
+
+## Weaver Protocol Buffers
+
+To test the Rust build, you need to run either one of the following:
+
+`yarn lerna run proto:protoc-gen-rust` (from the project root)
+
+or a regular cargo build from the core-api package directory, e.g.:
+
+```sh
+cd packages/cactus-core-api/
+cargo build
+```
+
+If you prefer to use `make` that also works from the package directory:
+
+```sh
+cd packages/cactus-core-api/
+make
+```
+
+The protocol buffer to Rust compilation is not yet part of the project build by
+default because we need some time to include the Rust toolchain in the project-wide
+list of build dependencies for developers.